Industrially developed countries generate billions of tons of air pollutants, a great part accounts to the combustion of coal, oil, and gasoline in electric power plants. Other major air pollution sources include petroleum refineries, cement plants and petrochemical plants.
Hg (mercury) is a naturally occurring metal used in a wide variety of applications ranging from the production of household bleach to the mining of gold. Hg is released into the environment either directly to water via waste systems, or much more commonly, directed to the atmosphere. It is this atmospheric pathway that is largely responsible for Hg contamination in the environment. The combustion of coal for energy production, and incineration of municipal and medical wastes, produces the majority of Hg deposited onto the watersheds.
In the atmosphere, Hg undergoes a wide variety of chemical transformations, eventually settling to the landscape as Hg attached to particulate matter such as soot. Once on the ground, Hg migrates through watersheds, arriving eventually into receiving waters (e.g. wetlands and lakes). Hg that is moving through watersheds is subject to myriad chemical transformations, and these are often biologically mediated. The most important of these biological transformations is the generation of methyl-mercury (Me-Hg). Me-Hg is a highly toxic form of Hg, which is easily assimilated into tiny planktonic organisms at the base of aquatic food chains. Through the processes of bio-magnification, minute concentrations of Me-Hg are passed up food chains, increasing to levels that pose a significant threat to those organisms feeding at the top of the aquatic food web. Organisms which are at risk of Me-Hg exposure include top-level carnivorous fish such as walleye, fish-eating birds such as eagle and loons, and, of course, humans. It is noteworthy that some fish species are better at getting rid of their Me-Hg burden than others.
The increasing environmental awareness during the last decades has led in many countries to governmental regulations, enforcing standards for maximum air pollutants emission on power plants and industries, in order to achieve air quality standards for various hazard materials. For example, in the U.S., the Clean Air Act of 1967 as amended in 1970, 1977, and 1990 is the legal basis for air-pollution control throughout the U.S.
The need to obey these regulations has led to the development of diversified methods for controlling and reducing the emission of air pollutants. In general, these methods include removing the hazardous material before it is used (for example, using low-content sulfur coal) or removing the pollutant after it is formed. For some air pollutants, only methods of the second class can be employed.
The present invention relates to methods of the second class.
Industrially emitted pollutant gases can be entrapped by liquids or solids traps that adsorb the harmful gases before they are released into the atmosphere. These traps are usually in the form of tower tanks contactors through which the waste gas is passed upward while the liquid, or a slurry, i.e. a mixture of liquids and solids, is descending downward. The agent which absorbs the pollutants and prevents their emitting into the atmosphere is called the scrubbing agent. When this agent is a liquid, it is sometimes the practice to fill the tower with inert particles in order to increase the contact surface between the scrubbing agent and the waste gas and to increase the residence time of the gas inside the reactor. In such cases it is also the practice to circulate the liquid scrubbing agent through the reactor until it is loaded with the pollutants.